1. Field of the Invention
The present invention relates to an optical element used in optical devices such as video cameras, digital cameras, still video cameras, copiers, and so on.
2. Related Background Art
Various proposals have been made heretofore about optical systems making use of reflecting surfaces such as concave mirrors, convex mirrors, and so on. FIG. 7 is a schematic diagram of a so-called mirror optical system consisting of one concave mirror and one convex mirror.
In the mirror optical system in the same figure, object light 104 from an object is reflected by the concave mirror 101 toward the object while being coverged, then is reflected by the convex mirror 102, and thereafter is focused on the image plane 103. Numeral 105 designates the optical axis of this optical system.
This mirror optical system is based on the structure of the so-called Cassegrain reflecting telescope and is designed for the purpose of decreasing the entire length of the optical system by folding optical paths of a telescope lens system of long entire length composed of refracting lenses by use of two reflecting mirrors opposed to each other.
In the objective systems for telescopes, there are a lot of known types of optical systems for decreasing the entire length of the optical system by use of a plurality of reflecting mirrors, in addition to the Cassegrain type, for the same reason.
As described, the compact mirror optical systems have been constructed heretofore by efficiently folding the optical paths by use of the reflecting mirrors instead of lenses in the taking lenses of long entire length.
In these reflective optical systems, optical components need to be assembled with accuracy in order to achieve desired optical performance. Particularly, since errors in relative position accuracy of the reflecting mirrors strongly affect the optical performance, it is important to accurately adjust the position and angle of each reflecting mirror.
A solution to this issue is a proposal of a method of avoiding assembly errors of the optical components during assembly by constructing the mirror system with reflectors of one block.
For example, in the case of non-coaxial optical systems, the optical systems in a well-corrected state of aberration can be constructed by introducing the concept of the reference axis and making constituent surfaces of rotationally asymmetric, aspherical surfaces; Japanese Patent Application Laid-Open No. 9-5650 describes the design method thereof and Japanese Patent Applications Laid-Open No. 8-292371 and Laid-Open No. 8-292372 describe design examples thereof.
Such non-coaxial optical systems are called offaxial optical systems (which are optical systems defined as optical systems including curved surfaces (offaxial surfaces) the normal to which at an intersection with the reference axis is off the reference axis, where the reference axis is taken along a ray passing the center of the image and the center of the pupil, and in which the reference axis is in the folded state).
In the offaxial optical systems, the constituent surfaces are normally not coaxial and no eclipse occurs even at the reflecting surfaces, which facilitates construction of the optical systems using the reflecting surfaces. These optical systems also have such features that routing of optical paths is relatively free, an integral optical system can be formed readily by a method of integral molding of the constituent surfaces, and so on.
FIG. 8 is a schematic diagram to show an embodiment of the reflecting optical system disclosed in Japanese Patent Application Laid-Open No. 8-292371.
In FIG. 8, numeral 10 designates an optical element having a plurality of curved, reflecting surfaces, which is constructed of a transparent body of glass or the like. The optical element 10 has a concave refracting surface (entrance surface) 11 of negative refractive power, four reflecting surfaces comprising a concave mirror 12, a reflecting surface 13, a reflecting surface 14, and a concave mirror 15, and a convex refracting surface (exit surface) 16 of positive refractive power, which are formed in surfaces of the optical element 10 and in order named according to passage of rays from the object.
Numeral 2 denotes a stop (entrance pupil) placed on the object side of the optical element 10, 3 an optical corrector such as a quartz low-pass filter, an infrared cut filter, or the like, and 4 a final image plane, in which an image pickup surface of an image pickup device (imaging medium) such as CCD or the like is located. Numeral 5 indicates the reference axis of the photographing optical system (which is an axis passing the center 6 of the stop 2 and normally entering the center of the image plane 4). Numeral 6 represents the center of the stop 2.
The two refracting surfaces both are rotationally symmetric, spherical surfaces and all the reflecting surfaces are surfaces symmetric with respect to only the YZ plane.
The imaging action will be described next. The light 1 from the object is regulated in the amount of incident light by the stop 2 and thereafter is incident to the entrance surface 11 of the optical element 10. The light is reflected by the surfaces 12, 13 and thereafter once forms an image near the surface 13. Then the light is reflected successively by the surfaces 14, 15 and emerges from the exit surface 16. The light again forms an image on the final image plane 4 through the optical corrector 3. The object rays form the intermediate image near the surface 7 and pupil rays form an intermediate image between the surface 14 and the surface 15.
In this embodiment the direction of the reference axis of incidence to the optical element 10 is parallel and identical to the direction of the reference axis of emergence therefrom. The reference axis including incidence and emergence all lies on the plane of the drawing (the YZ plane).
As described, the optical element 10 functions as a lens unit having desired optical performance and positive refractive power as a whole, based on the refractive powers of the entrance and exit surfaces and the refractive powers of the curved reflectors therein.
The invention disclosed in Japanese Patent Application Laid-Open No. 8-292371 decreased the effective diameter of the optical system even in the reflecting optical system of wide angle of view by the structure in which the stop was placed closest to the object in the optical system and in which the object image was formed at least once in the optical system, and also decreased the entire length of the optical system in the predetermined direction by bending the optical paths in the optical system into the desired shape by the structure in which the reflecting surfaces forming the optical element were provided with their respective, appropriate, refractive powers and in which the reflecting surfaces forming the optical element were placed in the non-coaxial relation.
Japanese Patent Application Laid-Open No. 8-292371 also discloses an example of the reflecting optical system wherein the entrance reference axis and the exit reference axis are not within a common plane, through free routing of optical paths.
FIG. 9 is a schematic diagram of the main part of such a reflecting optical system as disclosed in Japanese Patent Application Ladi-Open No. 8-292371. In FIG. 9, numeral 10 designates an optical element having one reflecting plane and a plurality of curved, reflecting surfaces, which is constructed of a transparent body of glass or the like. The optical element 10 has a convex refracting surface (entrance surface) R2 of positive refractive power, six reflecting surfaces of a reflecting plane R3, a concave mirror R4, a convex mirror R5, a concave mirror R6, a reflecting surface R7, and a concave mirror R8, and a convex refracting surface (exit surface) R9 of positive refractive power, which are arranged in the order named according to passage of rays from the object and in surfaces of the optical element 10. R1 denotes a stop (entrance pupil) placed on the object side of the optical element 10, and R10 the final image plane on which the image pickup surface of the image pickup device such as the CCD or the like is located. Numeral 5 represents the reference axis of the photographing optical system.
The two refracting surfaces both are rotationally symmetric, spherical surfaces and all the curved, reflecting surfaces are surfaces each having only one symmetry plane.
This optical element 10 can also be produced by integral molding with a mold, including the entrance refracting surface R2, the reflecting plane R3, the curved, reflecting surfaces R4 to R8, and the exit refracting surface R9. However, the molding needs to take account of the shape of the mold, the direction of draft, the draft angle, and so on, and the shape of the optical element (particularly, the relation between effective diameters of R5 adjacent to the reflecting plane and the entrance refracting surface R2, etc.) is thus limited in order to satisfy those conditions.
Particularly, where the stop (entrance pupil) R1 is absent in front of the optical element, the optical effective diameter of the R1 surface is larger than those of the R2 surface and subsequent surfaces and the R1 surface etc. become large in comparison with the optical element. As a result, it will be difficult to integrally mold the optical element in that case.
In the cases wherein the entrance reference surface and the exit reference surface are not within a common plane, the optical element is of the asymmetric shape as illustrated in FIG. 9. Then the pressure will be exerted in nonuniform distribution on a molded product during molding and shrinkage will be nonuniform during cooling of the molded product.
An object of the present invention is to facilitate assembly and production of optical elements having a plurality of reflecting surfaces consisting of curved surfaces and/or planes in surfaces of a transparent body, and thus provide an optical element with high optical performance and an imaging apparatus using it.
Another object of the present invention is to permit construction of optical elements even in the structure in which the entrance reference axis and the exit reference axis are not within a common plane, without any restrictions, and thus provide an optical element advantageous in terms of molding accuracy as well and an imaging apparatus using it.
For accomplishing the above objects, an optical element of the present invention comprises a first optical member which is a transparent body having two refracting surfaces and a reflecting, curved surface symmetric only with respect to one symmetry plane and a reference axis of which is present in the symmetry plane; and a reflecting surface and the reference axis of which is not present in the symmetry plane. One refracting surface of the second optical member is coupled to one refracting surface of the first optical member. In addition, the reference axis is defined by a ray passing an image center and a pupil center of an optical system including the optical element of the present ivention.